TUESDAY, June 26 (HealthDay News) -- If you need an excuse to pour yourself that second cup of coffee, read on. Moderate, daily coffee drinking may be good for your heart -- to a point, a new study suggests.

"We found that moderate consumption may, in fact, be protective," said Elizabeth Mostofsky, study lead author and a research fellow at the Harvard School of Public Health in Boston. "There are many factors that can contribute to a person's risk of heart failure, but moderate coffee consumption probably isn't one of them," she added.

The research was published June 26 in the journal Circulation Heart Failure.

In heart failure, the heart has difficulty pumping enough blood to meet the body's needs. It can be caused by such health threats as coronary artery disease or high blood pressure. About five million people in the United States have heart failure, and it contributes to 300,000 deaths annually, according to the National Institutes of Health.

The study authors concluded that about two typical American 8-ounce caffeinated cups of coffee daily (the equivalent of four northern European servings) may prevent heart failure, decreasing risk by up to 11 percent.

But drinking too much coffee -- more than four or five U.S. coffee shop-sized cups a day -- could raise the risk of developing the heart problem.

For their analysis, the researchers reviewed five large studies of coffee consumption and heart failure risk published between 2001 and 2011. The studies included 6,522 heart failure events among 140,220 people in Sweden and Finland.

The study did not distinguish between caffeinated and decaffeinated coffee, but caffeinated coffee tends to be the norm in those two northern European countries.

The new research adds to a range of recent studies that have shown that coffee may protect against some illnesses, including type 2 diabetes, Parkinson's disease, liver cancer and cirrhosis of the liver, and might improve exercise performance.

While health experts still warn that people who are pregnant, have difficulty controlling their blood pressure or blood sugar, or experience palpitations or jitteriness should drink just a little java or none at all, the researchers behind the new study say most people should feel free to enjoy coffee -- within limits.

The reason for the heart-protective effect is not fully understood, the researchers said. People who regularly drink coffee typically develop tolerance to coffee's caffeine, which may mean they're less likely to feel its effects. That may put them at decreased risk of high blood pressure, Mostofsky said. Also, antioxidants in the beverage may protect cells from damage, she said.

Some experts expressed some caution about the new study.

"The evidence is not strong enough to recommend that people should drink coffee to protect themselves," said Dr. Arthur Klatsky, an adjunct investigator with the Division of Research at Kaiser Permanente Medical Center in Oakland, Calif. Klatsky was not involved in the study.

Klatsky, who has done research on the relationship between heart rhythm and coffee, said coffee drinking is a lifestyle factor. "It could be that people who drink coffee also exercise more or have better diets," he said.

The bottom line, he said, is that "people should not feel they should avoid coffee if they're at risk for heart failure."

The study was supported by grants from the U.S. National Institutes of Health.